The invention concerns an X-ray optical configuration for irradiation of a sample with an X-ray beam having a line-shaped cross-section, wherein the configuration contains an X-ray source and a beam-conditioning X-ray optics.
A configuration of this type is disclosed e.g. in the leaflet by Bruker AXS “Super Speed Solutions” (2003 Bruker AXS, Karlsruhe).
In X-ray diffractometry (XRD), interferences (reflexes) are generated on three-dimensional periodic structures on an atomic scale (crystals) in accordance with Bragg's Law. The angular position of the reflexes and the intensity thereof contain important information about the atomic structure and microstructure of the substances to be examined.
Point sources are used in X-ray diffractometry for examining point-shaped objects, e.g. small crystals with an edge length of 10 to 100 micrometers, or for measurements with a position resolution of down to a few 10 square micrometers on relatively large sample surfaces such as semiconductor wafers.
Line sources, however, are used for examining relatively large sample surfaces. This is typical for the use of the Bragg-Brentano geometry for determining crystalline phases in a sample and also for high-resolution diffractometry and high-resolution reflectometry. The use of line sources usually has two advantages: firstly, the electrons from the cathode and therefore the current are distributed over a larger surface of the anode (e.g. 0.4×12 mm2 with a long fine focus tube). In this fashion, it is possible to typically operate at very high power, while preventing the anode from melting due to the heat load. The second advantage results from the fact that, with commercial metal ceramic tubes, the X-ray beam is normally extracted from the anode at an angle which is approximately 6°. For this reason, the visible focal spot is only 0.04×12 mm2. The size of 0.04 mm has the effect that the angular resolution obtained in the diffraction experiment is much better compared to similar point sources.
The X-ray tube of a size of 0.4×12 mm2 has a second X-ray permeable window at 90° relative to the line focus window. At an extraction angle of 6°, the focal spot has a size of 0.4×1.2 mm2. The X-ray beam flux has exactly the same magnitude as through the window for the line focus but the angular resolution of the experiment is considerably worse due to the larger extension of the focal spot in the x-direction.
However, there are also diffraction experiments such as e.g. texture or internal stress, in which cases the angular resolution is not decisive.
Point sources that provide a resolution that is comparable to line sources should therefore have a focal spot of approximately 0.04×0.04 mm2. These are microfocus sources which function, however, only at 50 W since the surface load with electrons would otherwise cause the anode to melt.
With a line focus, a larger amount of sample material additionally contributes to scattering in consequence of which a larger amount of the radiation is generated and the signal becomes larger, which again reduces the measuring time and/or improves the signal-to-noise ratio.
In order to be able to perform the whole range of measuring methods of thin layers, microstructures and nanostructures by means of X-ray diffractometry, the commercially available X-ray diffractometers must be converted between line focus and point focus sources. This conversion is extremely complex and time-consuming, since either the X-ray tube of glass ceramic tubes must be rotated, or the cathode, filament and direction of installation of rotating anodes must be changed. In correspondence therewith, the associated optics must be changed and readjusted, which is in most cases also complex. This obstructs, in particular, the use of microfocus sources or other brilliant X-ray sources.
The present invention enables the use of both point-shaped and line-shaped beam geometries without complicated and time-consuming conversion work.